This specification generally relates to image sensors. More specifically, the present specification provides variable adjustment of gain and exposure time for an image sensor array.
Complementary metal-oxide semiconductor (CMOS) active pixel sensor (APS) technology is an integrated circuit technology for electronically capturing images, therefore forming an “electronic film.” Unlike charge-coupled device (CCD) image sensors that have been used in the past to capture electronic images, the CMOS APSs use mainstream microelectronic fabrication processes to produce the sensor chips. The advantages of the CMOS APS technology are low cost, small size, and an ability to integrate a significant amount of very-large-scale-integrated (VLSI) electronics on-chip.
In the CMOS APS device, each pixel of an image sensor array has both a photodetector and also active transistor circuitry for readout of the pixel signal. The photodetector for each pixel in the image sensor array converts photons impinging on it to an electric charge via the photoelectric effect. The charge is integrated over a period of time long enough to collect a detectable amount of charge but short enough to avoid saturating storage elements. This period is analogous to a film exposure time (i.e. the shutter speed) and is called an integration time.
In general, photodetectors are more sensitive to light than film, which allows APS cameras to have the short integration times ranging from 1/30 to 1/20000 of a second. N In the image sensor array of the CMOS APS device, the integration time is the time between a reset of a given row and a subsequent read of the row. Since only one row can be selected at a time, the reset/read routine is sequential (i.e. row by row). This routine is referred to as a “rolling electronic shutter.” The integration time is adjusted by a fixed amount as a result of an image exposure analysis using an average intensity scheme or image histogram measurements.
The shutter width is the time between integration enable and readout. This can be of any size depending on the number of adjacent pixels that will have the same integration time. The shutter width can be shorter or longer than one row. The shutter width can also be adjusted by a fixed value to control the gain of an exposed area of a sensor array.
Some of the disadvantages of using constant adjustment values for the shutter width and the integration time include a slow adaptation time and a high quantization noise. The adaptation time is the amount of time it takes to adjust the integration times for the entire image sensor array. The adaptation time can be very long for a large sensor array with a large number of rows for a system with a fixed shutter width. For example, the adaptation time is more than one minute for a shutter width of one-half of a row and an integration time of 1/30 second for a 1000-by-1000 sensor array.
The quantization noise relates to an increment of the shutter width and the accuracy of the integration time. Both affect the visibility of an image contrast difference. The image contrast difference is a percentage change in the integration time and can be expressed as a ratio of the change in integration time over the integration time. The human eye can resolve the contrast difference on the order of about 2%. Thus, for example, change in shutter width from 10 rows to 11 rows will be visible since this is equivalent to a 10% contrast difference. However, the decrease in the shutter width to shorter than one row size to allow small contrast difference causes significant increase in the adaptation time.